Well, the idea is that those two grow up to be the ultimate wonder duo, and in some point Kacchan just start to compliment deku while they are fighting the villains and Deku is like ????????
Eventually, Compliments turns into “YOU WANT TO HAVE DINNER LATER?!” “HEY, WANT TO SEE THAT NEW MOVIE?!” and some casual talk while they are kicking ass. “OI, HOW IS THE OLD MAN?! (All might)” “MY DAD IS OKAY, THANK YOU FOR ASKING”
So they start to date (In the end is Deku who kiss katsuki first and ask him out, Kacchan is a little mad, but just accept it that he was still a little afraid to ask Deku because he don’t want to ruin what they have)
AFTER SOME MORE TIME AND A LONG BATTLE WITH A SPECIAL VILLAIN katsuki Ask in a normal conversation without shouting for first time “Want to marry me?” and deku don’t say “Kacchan” but “katsuki” and she is really happy because omg, after a tons of trouble and a lot of things, they finally understand each other and want to stay together
At least more or less that how I imagine it